powerpc: Drop CONFIG_MTD_M25P80 in 85xx-hw.config

Drop CONFIG_MTD_M25P80 that was removed in
commit b35b9a10 ("mtd: spi-nor: Move m25p80 code in spi-nor.c")

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1588394694-517-1-git-send-email-bmeng.cn@gmail.com